Age determination of free-living male black-throated blue warblers during the breeding season

Abstract

Male Black-throated Blue Warblers (Dendroica caerulescens) can be aged as yearlings (SY) or older (ASY) during the breeding season using binoculars. Significant differences in wing color of SY and ASY males, as revealed be spectrophotometric analysis of museum specimens, are apparent in free-living birds. Error rates in age determination in five held trials averaged 3.1% for ASY males and 0% for SY males (though sample sizes were small for yearlings). Age data from multiple investigators should not be combined for analysis unless individual error rates can be estimated. The statistical effects of error are especially critical when age ratios are calculated for small samples (n < 25).